Method of manufacturing magnetic materials



Patented July 13, 1943 Marnoo or MANUFACTURING MAGNETIC MATERIALS JacobLouis Snoek, Eindhoven, Netherlands;

, vested in the Alien Property Custodian No Drawing. Application m y 29,1941, Serial No. 395,880. 1 In the Netherlands June 12, 1940 8 Claims.(cr m -215i This invention relates to a method of obtaining improvedmagnetic properties of dispersioned-hardened alloys, fo'r example Ni-Al-F'e alloys with or without additions. Such alloys are particularlysuitable for permanent magnets.

The methods of heat treatment hitherto known for these alloys have thecommon feature that attention is only paid to the temperature to whichthe body is to be heat treated or the temperature it assumes in thecourse of the hardening process. 7

The heat treatment according to the invention on the contrary is basedon recognition of the fact that the progress of the dispersion hardeningprocess in a given small'regionof the material is not' only governed bythe heat treatment to which this region is subjected but also depends onthe degree of dispersion which at a given moment is reached in theimmediate proximity of the said region. r

According to this inventive idea such an intense temperature gradient ismaintained in the material in a given direction during the heattreatment that magnetic properties are obtained 7 which are moreadvantageous than those which could be obtained in the same alloys bymeans of heat treatments hitherto known. For this purpose, according tothe above idea the temperature gradient must be selected in such mannerthat the degree of dispersionin two small regions which are adjacenteach other in the di-..

rection of the temperature gradient is widely difierent.

More particularly, at the commencement of the dispersion process theexistence of such a temperature gradient will exercise a considerableinfluence on the progress of the dispersion process and thus on themagnetic properties eventually obtained. Since the exact temperature atwhich the dispersion starts is usually unknown but generally rangesbetween 700 and 1000 C. it is advantageous to maintain a sufficientlyintense temperature gradient in the material throughout this range oftemperature during the heat treatment. l

The use of the method according to the invention permits of obtainingparticularly, depending on the choice of the conditions, a higher valueof the residual magnetism, of the product (Bmmx, of the fullness factoror of the combination of these values than has been found possible bymeans of the methodshitherto known in connection with the same alloys(the term fullness factor is to be understood to mean here the value ofthe quotient In some cases it has been established that it is preferablethat after the heat treatment the material should'be magnetisedin oropposite to the direction in which the temperature gradient wasmaintained during the heat treatment since in these cases magneticproperties are obtained in the said direction which are more intensethan those obtained in the case of magnetisation having taken place inother'directions. By magnetization corresponding to the direction of thetemperature gradient is meant that the magnetic lines of force extend inthe same direction as the temperature gradient and that the north poleof the magnet maybe made toexist at either the -.high temperature or thelow temperature end of the temperature gradient. It has, however, notbeen established for certain that the magnetising method should be usedin any case that has to be dealt with. r

In order, that the invention may be clearly understood and readilycarried into efiect it will now be described more fully with referenceto some few examples.

Anw alloy having the composition 26.5% of nickel, 12.3% of aluminium,remainder of iron with normal impurities was cast in a sand mould in theform of rods being about'300mm. in length andhaving a diameter of 3 mm.In this case, if the size of the mould and the composition of themoulding sand have been effectively chosen a chilling occurs which issuch that the lowermost end of the rod is cooled considerably soonerthan the more upwardly situated part which receives heat for a furthertime from the material in the mould (temperature gradient).

A piece v mm. in length out of the central part of the rod had highlyfavourable magnetic properties (line 1 of Table I given below) aftercasting and magnetising in the direction of the temperature gradientwhich existed during chilling, it being even possible for saidproperties to be improved by ageing at about 650 C. (line 2 of Table I).For comparison mention is made in line 3 of Table I of the value whichcould beobtained in material of similar composition treated in theoptimum normal manner. From the results obtained it may be seen that aconsiderable improvement of the product BHmax and of the fullness factorhad occurred.

Table I In the above-mentioned example casting and hardening were unitedin one operation. It is, however, also possible for the castingoperation and the heat treatment to be brought about separately, as maybe seen from the following example.

A small rod of the same dimensions and the same composition as in thefirst-mentioned example but otherwise cast by any suitable method washeated as a whole in a. tube furnace up to 1200 C. and then verticallyimmersed in cold running water at a rate of about mm. per second. Inthis case, the part that emerges from. the water remains practicallyredhot up to the end, whereas the part under water is cold (temperaturegradient). As revealed by visual observation, the transition rangecovers a length of about 3 mm.

It may be observed that any part of the rod consequently traverses thetemperature interval from 1000 to 700 0., which is important inconnection with hardening, in less than 1 sec., presumably in about 0.1see. In the case of normal hardening (as occurs, for example, if the rodis immersed in the water horizontally) barely any hardening would occurat such a rate of cooling. In spite of this, the rod, after theabove-described treatment and hence in the presence of an intensetemperature gradient, manifests magnetic properties during hardening in'the direction of the temperature gradient, said properties being highlyfavourable for the said forms of steel and substantially equivalent tothe values obtained by the first-described casting method+ageing (cf.line 4 of Table I with line 2 of the same table).

As regards the obtainment of these favourable results the correct choiceof the cooling conditions is very important; once the principle beinggiven they can, however, be ascertained for any alloy by those skilledin the art.

It may be seen from the following example that the fullness factor incombination with a favourable (BH) max obtained by the use of the methoddescribed is essentially higher than 0.5, a figure which in the presentstate of the art is deemed the highest obtainable for nickel-aluminiumalloys without cobalt for which a (BH)max higher than 1.2 10 could nothitherto be obtained in practice.

Out of an alloy of the composition of 27.8% of Ni, 13.6% of Al, 2% ofCu, remainder of iron small rods were cast in the same manner asabove-described. As may be seen Table II given below these rodsmanifested abnormally high values of the fullness factor, even in thecast state, after magnetisation in the direction of the temperatureinterval adopted (line 1), values which still increase after ageing(line 2).

Table II 27.8% of Ni, 13.6% of Al, 2% of Cu, remainder of iron andimpurities Fl Heat treatment Br ll, (Bmmn (l) Rods 3 mm. cast in sandmould. 7. 400 l85 0.8240 0.00

(2) Ageing 1 hour 7,900 257 1.32-10 0. c5 (3) Unilateral cooling inwater 7, 900 135 0. 65-l0 0.61

from the (2) Normally treated Similar results as far as the fullnessfactor is concerned may be obtained by a special heat treatment in whicha temperature gradient is used (Table II, line 3).

The above-described methods of casting and of unilateral immersion canonly be applied to steels requiring comparatively quick cooling i. e. acooling rate of about 20 C./sec. or higher. In this case the heat of thematerial in the mould or the proper heat of the material suffices tobring about the required intense temperature gradient.

In connection with magnet alloys requiring slower cooling it isnecessary that during the cooling heat should be supplied on one side inorder that a temperature gradient may be maintained for a longer time.This may be carried into practice in various ways. One practical examplein which a "slow magnet steel is concerned is the following:

An alloy of the composition 15% of Ni, 8.5% of Al, 23% of Co, 0.75% ofSi, remainder of iron and impurities was cast in long rods having adiameter of 3 mm. These rods were slowly drawn from a furnace heated at1200 C. into another furnace kept at about 500 C. The rate of drawingwas about 1 cm. in 10 minutes, the temperature interval between 1200 and500 C. was traversed in about 1 hour.

It may be seen from the following Table III that a high residualmagnetism and a very high fullness factor are thus obtained. In line 2mention is made of the values obtained at a corresponding cooling ratebut without temperature gradient.

(mom.

Heat treatment It-H:

(1) Treated with temperature gradient 11,400 116 0.8840 0.60

without temperature gradient What I claim is:

l. The method of improving the magnetic properties of a magnetizablebody of an alloy hardenable by dispersion, comprising subjecting saidbody in a given direction to a temperature gradient of such intensity asto enhance the magnetizability of the body in the given direction, andmagnetizing said body in a direction corresponding to the direction ofthe temperature gradient.

2. The method of improving the magnetic properties of a magnetizablebody of an alloy hardenable by dispersion, comprising cooling said bodyfrom a temperature of at least 1000 C., subjecting said body to atemperature gradient in a given direction, maintaining said temperaturegradient at least during the temperature interval of from about 1000 C.to about 700 C., and magnetizing said body in a direction correspondingto the direction of the temperature gradient.

3. The method of improving the magnetic properties of a magnetizablebody of an alloy having as main constituents nickel, aluminum and iron,comprising cooling said body while maintaining in a given direction atemperature gradient of such intensity as to enhance the magnetizabilityof the body in the given direction, and magnetizing said body in adirection corresponding to the direction of the temperature gradient.

4. The method 01' improving the magnetic properties of a magnetizablebody of an alloy having as main constituents nickel, aluminum and iron,comprising cooling said body from a temperature of at least 1000,C., soas to bring about a temperature gradient in a given direction,maintaining said temperature gradient at least during the temperatureinterval of from about 1000 C. to about 700 C., and magnetizing saidatemperature gradient in a direction corresponding to the longitudinalaxis of the body and magnetizing said body in the direction of thelongitudinal axis of the body.

6. In the manufacture of a, magnetizable body, the process comprisingmolding a nickel-aluminum-iron alloy into a body of the desired shape,chilling different portions of said body at predetermined selected ratesso as to bring about a temperature gradient in a given direction throughthe cooling body and magnetizing said body in a direction correspondingto the direction of the temperature gradient.

7. The method of improving the magnetic properties of a magnetizablebody of an alloy having as main constituents nickel, aluminum, cobaltand iron, comprising cooling said body while maintaining in a givendirection a temperature gradient of such intensity as to enhance themagnetizability of the'body in the given direction, and magnetizing saidbody in a directioncorresponding to the direction of the temperaturegradient.

8. The method of improving the magnetic properties of a magnetizablebody of an alloy hardenable by dispersion, comprising cooling the bodyfrom a temperature above the dispersion temperature of the alloy to atemperature belo the dispersion temperature of the alloy, maintaining ina given direction and during a temperature interval including thedispersion temperature of the alloy a temperature gradient of suchintensity as to enhance the magnetizability of the body in the givendirection, and magnetizing said body in a direction corresponding to thedirection of the temperature gradient.

JACOB LOUIS SNOEK.

